In urban areas where human activities are active, there are volatile organic compounds (VOCs) emitted notably in addition to the emissions of NOx, SO2 and carbon monoxide. These VOCs are emitted much more in urban areas than in the other areas. As for the VOCs, reactive hydrocarbons which react with OH radical are determinative of the strength of forming photochemical ozone in an urban area. While the reactive hydrocarbons are divergent in types, including methane, non-methane hydrocarbons (NMHCs), alcohols, and carbonyl compounds (ketones, aldehydes), the NMHCs which are highly reactive are especially important as being precursors of ozone.
The increase in the amount of emission of NMHCs as precursors of ozone in urban areas in recent years is not only recognized to accelerate the problem of air pollution in those areas but also is now pressingly to raise the important issue of adversely affecting the global atmospheric environment.
The strength or concentration of air pollutants has so far been measured using sensors which measure the individual concentrations of these NOx, SO2, CO and VOCs respectively.
As for NMHCs, however, it is considered that there exist at least 200 types which are counted from an olefin to aromatic hydrocarbons, and measuring the individual concentrations of all these reactive hydrocarbons to estimate their rates of reaction with OH radicals and to estimate from their each rates the concentration or strength of ozone to be generated photo-chemically would give rise quite possibly to a large error despite the large efforts which this estimation should entail.